Method for NOx reduction of externally-ignited, explosion, internal combustion engines

ABSTRACT

In a method for reducing NO x  in the exhaust gas of an externally ignited, explosion-type internal combustion engine that is operated at a lean fuel/air ratio of λ&gt;1 and has a motor control device for controlling λ, the λ value is slidingly controlled as a function of an average mass temperature in the combustion chamber of the externally ignited, explosion-type internal combustion engine, wherein for a low average mass temperature λ is decreased and for a high average mass temperature λ is increased such that the internal combustion engine is operated in any operational state close to the misfiring limit, and thereby, with the most minimal NO x  emission possible.

[0001] The present application is a continuation-in-part of pending U.S.application Ser. No. 09/210,514, filed Dec. 11, 1998.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for reducing NO_(x) ofexternally ignited, explosion-type internal combustion engines in whichthe externally ignited, explosion-type internal combustion engine isoperated at an air ratio of λ>1 in a lean mode and the λ value of thefuel/air mixture is controlled by an engine control device.

[0003] For the purpose of reducing the NO_(x) values in the exhaust gasof externally ignited, explosion-type internal combustion engines it isknown to operate them with a lean mixture at λ>1. Because of the highaverage mass temperature in the combustion chamber during stationaryoperation, high λ values must be selected in order to achieve low NO_(x)emission.

[0004] Difficulties arise during non-stationary operation, for example,during acceleration from low engine rpm (revolutions per minute), whenthe average mass temperature in the combustion chamber is lower thanduring stationary operation and the misfire limit moves toward lower λvalues.

[0005] It is therefore an object of the present invention to control theλ value of an externally ignited, explosion-type, or gas, internalcombustion engine that is always operated at a lean fuel/air ratio ofλ>1, where λ is controlled by an engine control device of the internalcombustion engine, such that, for the purpose of NO_(x) reduction, theengine is operated close to the misfire limit.

SUMMARY OF THE INVENTION

[0006] Inventively, this object is solved in that the λ value isslidingly controlled as a function of the average mass temperature inthe combustion chamber of the externally ignited, explosion-type,internal combustion engine such that for low average mass temperaturethe λ value is decreased and for high average mass temperature the λvalue is increased so that the internal combustion engine, under anyoperating condition, is operated close to the misfire limit, or in otherwords, operates in any operational state close to faultless engineoperation.

[0007] Since the control of λ is based on the average mass temperature,the fuel/air mixture can be made so lean or so rich that the misfirelimit is almost reached but not surpassed. The NO_(x) reduction processis thus expanded to its physically possible limit.

[0008] As noted above, the method of the present invention is concernedspecifically with a retiming of the firing or ignition point ofexternally ignited, internal combustion engines, for example, gasengines, also commonly known as “explosion” engines. With gas orexplosion engines, the fuel-air mixture λ can only be varied within verynarrow limits, specifically, between an upper and lower ignition limit.

[0009] The present invention proposes to lower the lower ignition limitto such a point that no misfiring occurs, but to lower the lowerignition limit by lowering the combustion temperature, as determined bythe exhaust gas. As a result of this lowering of the combustiontemperature, a noticeable decrease of the NO_(x) concentration results.The average mass temperature is used as the control maximum for λ. Witha hot or warm engine, λcan be greater than with a cold engine.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0010] An advantageous possibility for determining the average masstemperature is as follows. The exhaust gas temperature is measured by atemperature sensor and the measured exhaust gas temperature, which is ameasure of the average mass temperature, is employed in the controllingstep. The exhaust gas temperature reflects the average mass temperatureso that the exhaust gas temperature can be used as a substitute for theaverage mass temperature. The exhaust gas temperature can be easilydetermined by a temperature sensor and can be used in the engine controldevice for controlling the λ value.

[0011] Another advantageous embodiment is as follows. With the aid ofthe engine control device, the exhaust gas temperature is calculatedbased on one or more operational parameters of the externally ignited,explosion-type, internal combustion engine. The operational parametersinclude load, engine rpm, coolant temperature, and atmospherictemperature etc. The calculated exhaust gas temperature, that is afunction of the average mass temperature, is then used in the step ofcontrolling. Since the exhaust gas temperature is dependent on theaverage mass temperature, the calculated exhaust gas temperature can beused as a substitute control signal by the engine control device for thepurpose of controlling the λ value.

[0012] According to another embodiment of the invention, the cylinderpressure curve of the externally ignited, explosion-type, internalcombustion engine is measured and, based on the measured curve, theaverage mass temperature is calculated. A pressure sensor is used tomeasure the pressure curve of the cylinder and then the average masstemperature is calculated therefrom and employed for controlling the λvalue.

[0013] The invention is based on the recognition that the λ value is nota fixed value, but that the critical limit for λ in the dynamic range ofthe engine has no set value. That is, the critical limit for λ is asliding limit that depends on the average mass temperature in thecombustion chamber.

[0014] The problem is that the currently known temperature sensors aretoo slow to make direct use of the average mass temperature for asliding control of the maximum λ value possible according to physicalprinciples. According to the methods of the prior art, it is necessaryto lower λ in the dynamic range of operation in order to preventmisfiring. The lowering of the λ value, however, results in increasedNO_(x) concentrations in the exhaust gas.

[0015] The inventive method suggests treating the λ value as a functionof the average mass temperature of the fuel/air mixture to be compressedwithin the combustion chamber. Since this average mass temperaturecannot be determined directly, it is inventively suggested to determinethe average mass temperature based on measurable parameters, forexample, by measuring the exhaust gas temperature with the aid of atemperature sensor and to then either calculate the average masstemperature with the aid of the engine control device as a function ofthe measured exhaust gas temperature and use the calculated result forcontrolling the maximum allowable λ value or to use the exhaust gastemperature as a substitute for the average mass temperature foradjusting the λ value.

[0016] Rather than measuring the exhaust gas temperature, a furtheroption suggests calculating the exhaust gas temperature based onparameters such as load, engine rpm, coolant temperature and atmospherictemperature, with the aid of the engine control device, and determiningthe average mass temperature to thereby control the λ value based on thedetermined exhaust gas temperature.

[0017] A further option for a sliding control of the λ value is tomeasure the cylinder pressure curve by a pressure sensor. With the aidof a combustion analysis the average mass temperature can be calculatedand the λ value can be controlled.

[0018] The inventive method allows control of the λ value in the dynamicoperational ranges of the externally ignited, explosion-type combustionengine up to the misfire limit of the engine without ever surpassing themisfire limit, such that the engine is in operated in any operationalstate close to faultless engine operation The maximum exploitation ofthe theoretically possible spectrum of the λ value ensures aminimization of the pollutant component in the exhaust gas, especiallyof NO_(x).

[0019] As previously noted, the present invention relates specificallyto a retiming of the firing or ignition point of externally ignited,internal combustion engines, for example, gas engines, also commonlyknown as “explosion” engines. With gas or explosion engines, thefuel-air mixture λ can only be varied within very narrow limits,specifically, between an upper and lower ignition limit.

[0020] The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What is claimed is:
 1. A method for reducing NO_(x) in the exhaust gasof an internal combustion engine, said method comprising the step of:slidingly controlling λ as a function of an average mass temperature inthe combustion chamber of the externally ignited, explosion internalcombustion engine, wherein for a low average mass temperature λ isdecreased and for a high average mass temperature λ is increased suchthat the externally ignited, explosion internal combustion engine isoperated in any operational state close to faultless engine operation,wherein said internal combustion engine is an externally ignited,explosion internal combustion engine that is operated at a lean fuel/airratio of λ>1, wherein λ is controlled by an engine control device.
 2. Amethod according to claim 1, further comprising the step of measuringthe exhaust gas temperature by a temperature sensor and employing theexhaust gas temperature, which is a measure of the average masstemperature, in the step of controlling λ.
 3. A method according toclaim 1, further comprising the step of calculating with the aid of theengine control device the exhaust gas temperature based on one or moreoperational parameters of the externally ignited, explosion internalcombustion engine, said operational parameters selected from the groupconsisting of load, engine rpm, coolant temperature, and atmospherictemperature, and employing the exhaust gas temperature, which is ameasure of the average mass temperature, in the step of controlling λ.4. A method according to claim 1, further comprising the step ofmeasuring a cylinder pressure curve of the internal combustion engineand calculating the average mass temperature based on the cylinderpressure curve.